msglist [nfc]: Drop theme from BackgroundData

Now that MessageList gets the theme from the ThemeContext,
the `theme` property can be removed from BackgroundData.

Author: BigHeadCreations

src/webview/backgroundData.js

@@ -12,7 +12,6 @@ import type {
   PerAccountState,
   Subscription,
   Stream,
-  ThemeName,
   UserId,
   User,
   UserOrBot,
@@ -63,7 +62,6 @@ export type BackgroundData = $ReadOnly<{|
   streams: Map<number, Stream>,
   subscriptions: Map<number, Subscription>,
   unread: UnreadState,
-  theme: ThemeName,
   twentyFourHourTime: boolean,
   userSettingStreamNotification: boolean,
   displayEmojiReactionUsers: boolean,
@@ -96,7 +94,6 @@ export const getBackgroundData = (
   streams: getStreamsById(state),
   subscriptions: getSubscriptionsById(state),
   unread: getUnread(state),
-  theme: globalSettings.theme,
   twentyFourHourTime: getRealm(state).twentyFourHourTime,
   userSettingStreamNotification: getSettings(state).streamNotification,
   displayEmojiReactionUsers: getSettings(state).displayEmojiReactionUsers,